Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may be composed of both particulate matter and gaseous compounds, such as NOx.
Due to increased attention on the environment, exhaust emission standards have become more stringent, and the amount of particulate matter and/or NOx emitted into the atmosphere from an engine may be regulated depending on the type of engine, size of engine, and/or class of engine. In response to these increasingly stringent regulations, engine manufacturers have been investigating various technologies for reducing the emissions from engines. One such technology for potentially providing reduced emissions includes the use of an HCCI engine.
In a typical diesel engine, the combustion process is initiated by the direct injection of fuel into an environment of highly compressed air within a combustion chamber. The fuel ignites almost instantaneously upon contact with the compressed air, and produces a diffusion flame or flame front extending along the plumes of the injected fuel. With this type of combustion process, relatively hot and relatively cool regions can coexist within the combusting fuel. This variation in combustion temperature can contribute to the production of undesirable emissions products. For example, nitrogen oxides may be formed in the hot peripheral zones while soot may be formed in the cooler areas.
Combustion in an HCCI engine, on the other hand, takes place spontaneously and homogeneously without flame propagation. In the HCCI engine, fuel is homogeneously premixed with air, but with a high ratio of air to fuel. For example, an injection system may inject into a combustion chamber a metered dose of fuel. During the ensuing moments, fuel and air throughout the combustion chamber have time to mix, forming a nearly uniform mixture. As the piston nears top dead center (TDC) of the compression stroke, compression heating of the piston within the cylinder causes this mixture to auto-ignite (spontaneously combust). The resulting spontaneous burn produces a flameless energy release in a large zone almost simultaneously.
In this type of combustion, there is no flame front and a substantially homogeneous temperature exists throughout the combustion chamber. As a result, less NOx and particulate matter emissions are produced, as compared to a typical spark ignited or diesel engine.
One challenge associated with HCCI engines relates to combustion timing control. Particularly, specific quantities of fuel and air in the combustion chamber must be precisely maintained in order to ensure auto-ignition at intended times. Supplying appropriate amounts of fuel and air to the combustion chamber and maintaining a desired combustion timing become especially challenging under changing engine operating conditions. Unlike traditional engines whose combustion timing can be controlled by varying the initial conditions of a combustion event (e.g., injection amount, injection timing, etc.), the combustion event in an HCCI engine depends on both the initial conditions and the ongoing conditions in the combustion chamber. To provide a desired combustion timing, in response to an operator changing the position of an accelerator, for example, the HCCI power system must ascertain initial cylinder conditions and then estimate temperature later in the cycle based on measured cylinder pressures. Any inaccuracy in this predictive element can lead to transient control problems in an HCCI engine.
To improve the operating characteristics of an HCCI engine-based power system, especially under transient operating conditions, there exists a need for a more accurate predictive methodology. For example, the capability to accurately model the characteristics of a future combustion event may enable an HCCI engine control system to maximize the probability that the combustion event occurring in the next engine cycle meets a desired set of performance characteristics.
At least one predictive control system has been developed in an attempt to accurately control the operation of an engine subject to transient changes of target outputs. Specifically, international patent publication no. WO 03/065135 (“the '135 publication”) discloses a control system in which a predictive model is employed to predict the outputs of the system in response to candidate new values for the inputs of the system. Based on the predicted response, the control system of the '135 publication determines inputs to the system that will cause the system outputs to match the target outputs as closely as possible.
While the system of the '135 patent may be capable of selecting an optimum candidate set of inputs based on a predictive model, the system of the '135 patent includes several shortcomings. For example, the system of the '135 patent relies exclusively upon predictive models (e.g., lookup tables including experimentally obtained data) that use interpolation to generate candidate sets of input values. This technique may be incapable of providing a desired level of accuracy under all conditions. Further, the system of the '135 publication is specifically aimed at the operation of exhaust gas recirculation (EGR) systems in traditional diesel engines including variable geometry turbochargers. The control system of the '135 patent may be unsuitable for providing the predictive capability necessary for operating an HCCI engine-based power system.
The presently disclosed systems and methods are directed to overcoming one or more of the problems set forth above.